1. Field of the Invention
This invention relates to isolation amplifiers, i.e. amplifiers having an input section which is galvanically isolated from all other portions of the equipment. More particularly, this invention relates to such isolators wherein the input and output sections are coupled by a single transformer serving the dual functions of supplying energizing power to the input section and carrying signal information to the output section.
2. Description of the Prior Art
A wide variety of isolators have been employed in the past for applications such as developing measurement signals for industrial process control systems, making medical measurements of the human body, and so on. A number of such isolators have employed a single transformer for effecting non-conductive coupling between the input section and other portions of the isolator. For example, U.S. Pat. No. 3,988,690 shows a design wherein such a single transformer includes three separate windings. In that arrangement, the power drawn through the transformer by the input section varies in accordance with the applied input signal and this power variation is detected in the output to produce a corresponding output signal.
U.S. Pat. No. 4,066,974 is directed to an arrangement which uses a transformer limited to only two windings, i.e. a single primary winding and a single secondary winding. The secondary winding is connected alternately to a d-c voltage and to a high-impedance. The d-c voltage connection couples energizing power through the transformer to develop a d-c power supply voltage in the input section, and the following high impedance connection produces a reverse-polarity flyback pulse which is modulated by the signal applied to the input section. The magnitude of the flyback pulse is detected in the output section to produce a corresponding output signal. A subsequent development, described in U.S. Pat. No. 4,152,660, adds to U.S. Pat. No. 4,066,974 the concept of utilizing the d-c power pulse in the transformer to produce a steady d-c voltage in the output section for energizing amplifier circuitry in that output section.
Experience with devices such as described in the above patents has shown that all suffer from important disadvantages, particularly with respect to performance characteristics such as drift of output signal with changes in temperature, non-linearity of the relationship between input and output, and changes in gain with temperature.